The presence of the glycoproteins haptoglobin and hemopexin in human plasma albumin (HPA) solutions were demonstrated to be responsible for the formation of polymers and aggregates during heat treatment for 10 h at 60°C. Apart from haptoglobin and hemopexin three other contaminating proteins were identified as transferrin, Gc‐globulin and β₂‐glycoprotein. During heat treatment the antigenicity of haptoglobin and hemopexin changed markedly and more than 90% of the antigens appeared as aggregates in the void volume during the following size chromatography. Without loss of albumin haptoglobin and hemopexin were removed from the HPA preparation by lectin (concanavalin A) affinity chromatography. The haptoglobin‐ and hemopexin‐depleted HPA preparation did not form aggregates or polymers during heat treatment.